Drosophila Polytene Chromosomes

Chromosomes can undergo several types of changes which fall into two classes. The first
type of change involves changes in chromosome number and is referred to as aneuploidy and
euploidy. How these type of changes can occur and their subsequent effect on phenotype will be
discussed in the next section. The chromosomal changes that we will discuss now alter the linear
order of the chromosome and occur because of deletions, duplications, inversions,
translocations and insertions of chromosomal DNA.

The analysis of these types of changes to a large part has been performed in genetic stocks
of the fruit fly, Drosophila melanogaster. The chromosomes of this species that are of particular
interest, are those found in the salivary glands of larvae. These tissues grow not by cell division
but by enlargement. During this enlargement the chromosomes also undergo replication. But this
replication is different than in other tissues because:

the homologous chromosomes remain synapsed, that is paired throughout the duplication; and

The banding patterns of these chromosomes are distinct from metaphase chromosomes and have been associated with
specific genes. Metaphase chromosomes also have distinct bands, but these are thought to be
associated with genetically inactive heterochromatin DNA. The term used to describe these
chromosomes and similar chromosomes in other diptera species is polytene chromosomes and
these chromosomes have primarily been usedd to: